Voracious industrial fishing has sucked more than 90% of large predatory fish from the world's oceans in only 50 years, Canadian researchers have calculated.

In this week's Nature, biologists Ransom Myers and Boris Worm of Dalhousie University in Nova Scotia report their analysis of trawl surveys and catch data from four continental shelf and nine oceanic systems over a 47-year period.

These ecosystems contribute almost half of the planet's primary production, the researchers note, and sustain three-quarters of global fishery yields. However, they found that industrialised fisheries typically reduce the biomass of ecological communities by 80% within 15 years of exploitation.

Global rates of decline for large predatory fish were estimated by comparing data before and after trawling commenced on the shelves of the northwest Atlantic Ocean, Gulf of Thailand, and Antarctic Ocean for codfish, flatfish, skates and rays, and Japanese longlining started in the equatorial and southern oceans for tuna, billfishes, and swordfish.

Whether on the continental shelf or in the open oceans, Myers and Worm found that commercial fishing decimated the large predators very quickly.

"The Gulf of Thailand, for example, lost 60% of large finfish, sharks and skates during the first five years of industrialised trawl fishing," they wrote. "The highest initial rate of decline was seen in South Georgia [Antarctic Ocean], which has a narrow shelf area that was effectively fished down during the first two years of exploitation."

They suggest many scientists and managers would be unaware of these rapid declines because they usually occurred before surveys were undertaken, and because fish biomass was stabilised at artificially low levels by management schemes implemented after fishing had started.

One of the effects of removing the large predators from the top of marine food chains is to increase other fish species. However, the data shows that this compensatory effect was often reversed within a decade or so, due to bycatch mortality, and the fishery shifting to target those species as the predators disappeared.

"Our analysis suggests that the global ocean has lost more than 90% of large predatory fishes. Although it is now widely accepted that single populations can be fished to low levels, this is the first analysis to show general pronounced declines of entire communities across widely varying ecosystems," wrote Myers and Worm.

Australian innovation

Closer to home, Australian researchers are refining a genetic method to more accurately measure the size of fish populations by estimating the number of breeders, rather than the number caught - which may enable fisheries to avoid such drastic declines in the future.

Dr Jennifer Ovenden, of the Molecular Fisheries Laboratory in Queensland's Department of Primary Industries, measures the genetic drift (the random change in gene frequency from one generation to the next) by comparing the genes in a population of fish from one generation to the next. The smaller the number of fish that manage to pass their genes onto their offspring, the greater amount of genetic variation that can be detected. This technique enables researchers to calculate the number of breeding animals that are successful in leaving offspring to the next generation.

Ovenden first tested the method on the tiger prawn fishery of Moreton Bay in Queensland - of 650,000 prawns at spawning time in 2001, only 0.2% were successful in producing offspring for the next generation. Estimates of spawning stock size - and particularly if they vary from year to year - is the critical link in the information required by fisheries managers.

"Most assessment of fisheries uses stock-recruitment models that measure stock in one year, and measure recruitment in the next year. For a fishery under exploitation, there's a relationship between the two. If you reduce the stock size, you'll actually get less recruits," she said.

In theory, Ovenden's method can be applied to other fish species - although prawns are ideal because they live for one year and have no overlapping generations. Spanish mackerel, which lives for about seven years, is being considered as another commercial fish population to which the genetic method could be applied.

"People may ask, if the entire prawn population of Moreton Bay could be regenerated from just 0.2%, then why do we have to preserve the entire spawning population?" she told ABC Science Online. "The answer: it's impossible to know which one of those spawning adults is actually going to be successful."

"It underlines the need to protect the breeding population and their habitat as a whole, rather than using the data as an argument to increase harvest rates. We want to facilitate fisheries stock assessments by providing this innovative and very new source of data."